1. Field of the Invention
The present invention relates to an optical head having a bobbin and an optical lens and an optical pickup having the optical head.
2. Description of the Related Art
Recently, there has been a demand to make an optical disc storage medium higher in density. Therefore, in optical disc apparatuses, research and development have been underway for shortening the wavelength of a light source and for enlarging the numerical aperture (NA) of a reproduction optical system. Further, it is desired to increase the transfer rate of data in such a reproduction optical system.
When shortening the wavelength of the light source and enlarging the NA of the reproduction optical system, in addition to the fact that the size of an optical spot becomes small, the focal depth becomes shallower, so it is desired to reduce the focus servo error. Further, a data recording width (track width) of the optical disc storage medium becomes narrower, so it is desired to reduce the tracking servo error.
For increasing the data transfer rate, it is desired to make the bandwidth of an actuator used for the focus servo and tracking servo higher. As a result, two improvements are desired for servo characteristics: reduction of the remaining amount of error and raising the bandwidth. An actuator can be improved in its servo characteristics by reducing the weight of the actuator.
FIG. 1 is a view of the configuration illustrating an optical head and show a schematic cross-section of the optical head.
This optical head 100 has a bobbin 10 and an optical lens 20.
At an outer circumference of the bobbin 10, a convex part 12A is formed along one surface (upper surface) 11A of the bobbin 10 and a convex part 12B is formed along the other surface (lower surface) 11B of the bobbin 10.
A center hole 10H is formed at the bobbin and a center axis of the center hole 10H is perpendicular or approximately perpendicular to the upper and lower surfaces 11A and 11B of the bobbin 10.
In the bobbin 10, a coil 13 is wound at a concave part between circumferential edges formed by the convex parts 12A and 12B. By arranging magnets outside of the coil 13 and supplying a driving current to the coil 13, it is possible to move the bobbin 10 and the optical lens 20 as one part.
The optical lens 20 has a convex part 21 functioning as a convex lens and a flat part 22 positioned around this convex part 21. A thickness of a substrate 24A at the flat part 22 is constant or approximately constant and is smaller than the thickness of the substrate 24A at the convex part 21. At the outer circumference of the convex part 21 is formed a groove 29 called a “trench” when forming the convex part 21 by etching. This groove 29 clearly separates the convex part 21 and the flat part 22.
A circumferential edge part of the surface of the flat part closely contacts the lower surface 11B of the bobbin 10. The convex part 21 is placed so as to fit into the center hole 10H of the bobbin 10.
The center axis of the center hole 10H of the bobbin 10 and an optical axis of the optical lens 20 coincide or substantially coincide.
The bobbin 10 is often formed by a plastic formed by injection molding for the purpose of lightening the weight and/or facilitating processing.
Because a plastic material has a large coefficient of thermal expansion, when mounting the optical lens 20 on the plastic material directly, due to the difference of the coefficient of thermal expansion between the bobbin 10 and the optical lens 20, thermal stress is often generated in the optical lens 20.
For example, when using silica-based glass as the optical lens 20, the coefficient of thermal expansion of the silica-based glass is about 0.4×10−6/° C., while when using polystyrene as the bobbin 10, the coefficient of thermal expansion of polystyrene is about 50×10−6/° C. Thus, there exists over a 100-fold difference in the coefficient of thermal expansion.
Since the groove 29 is formed around the convex part 21 of the optical lens 20, stress easily concentrates in the groove 29.
Further, there is a possibility of the optical lens 20 changing in refractive index due to the photoelastic effect arising from the thermal stress.